Spindle motor with magnetic shield for magnetic fluid seal

ABSTRACT

A magnetic disk drive device has a spindle motor for rotatingly driving magnetic disks in a predetermined direction, and a clamp member for mounting the disks on the spindle motor. The spindle motor has a hub rotatably supported through bearings and having a portion for mounting the magnetic disks, a rotor magnet carried by the hub, a stator disposed to oppose the rotor magnet, and magnetic fluid seal arrangement for preventing scattering of lubricant from the bearing means. The magnetic disk drive device further has a magnetic shield on the radially outer side of the magnetic fluid seal arrangement. One of the bearings rotatably supporting the hub may be charged with a lubricant having a comparatively low viscosity, while the other bearing may be charged with a lubricant having a comparatively high viscosity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic disk drive device forrotatingly driving a magnetic disk and a spindle motor for use in themagnetic disk drive device.

2. Description of the Related Arts

In general, a magnetic disk drive device has a spindle motor forrotatingly driving a magnetic disk in a predetermined direction and aclamp member for securing the magnetic disk to the spindle motor. Thespindle motor has a hub which is rotatably supported through a bearing,a rotor magnet mounted on the hub, and a stator disposed so as to opposethe rotor magnet. The magnetic disk is secured to the hub by the clampmember.

The spindle motor used in such a magnetic disk drive device is providedwith a suitable sealing means such as a magnetic fluid seal forpreventing grease from coming into the space receiving the magneticdisk. The magnetic fluid seal has an annular permanent magnet and polepieces which are mounted on both end surfaces of the permanent magnet. Amagnetic fluid is charged in the ends of the pole pieces.

In known magnetic disk drive device which employs a magnetic fluid seal,however, encounters with a problem in that magnetic fluxes from thepermanent magnet leaks into the space receiving the disk, causing errorsin reading and writing information. This problem caused by leakage ofmagnetic fluxes from the magnetic fluid seal is becoming more seriouswhen considered under the current trends for reduction in the size ofthe magnetic disk drive device and greater storage capacity of themagnetic disks.

Another problem encountered with the known magnetic disk drive deviceresides in that a loss of driving torque is increased when a lubricantof a comparatively large viscosity is used in a pair of ball bearingssupporting the rotational part of the spindle motor, requiring a greaterdriving electric current to be supplied to the spindle motor. Thisproblem would be obviated by the use of a lubricant having acomparatively small viscosity. Such a lubricant having a smallviscosity, however, exhibits a greater tendency of scattering and,hence, a greater tendency of invasion of the space where the disk isdisposed.

Furthermore, the known magnetic disk drive device requires a complicatedassembly process due to difficulty encountered in correctly locating thepair of bearings at predetermined positions and at a predeterminedspacing from each other.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a magnetic diskdrive device which is improved to reduce leakage of magnetic fluxes froma magnetic fluid seal incorporated in the device.

A second embodiment of the present invention is to provide a spindlemotor for use in a magnetic disk drive device, improved to effectivelyprevent invasion of the disk space by a lubricant while suppressing anyincrease in the loss of torque.

A third object of the present invention is to provide a spindle motorfor use in a magnetic disk drive device, improved to facilitate theassembly particularly in regard to the setting of bearings.

The above and other objects, features and advantages of the presentinvention will become clear from the following description when the sameis read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a portion of a first embodiment of themagnetic disk drive device in accordance with the present invention;

FIG. 2 is a sectional view of a portion of a second embodiment of themagnetic disk drive device in accordance with the present invention;

FIG. 3 is a sectional view of a first embodiment of the spindle motor inaccordance with the present invention;

FIG. 4 is an enlarged sectional view of the spindle motor shown in FIG.3, illustrating particularly bearing means incorporated in the spindlemotor; and

FIG. 5 is a sectional view of a second embodiment of the spindle motorin accordance with the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be more fully described with reference to thedrawings.

Magnetic Disk Drive Device

A first embodiment of the magnetic disk drive device of the presentinvention will be described with reference to FIG. 1.

Referring to FIG. 1, the first embodiment of the magnetic disk drivedevice of the present invention has a spindle motor generally denoted by2. The spindle motor 2 has a substantially disk-shaped bracket 4 and ahub 6 which is rotatable relative to the bracket 4. The bracket 4 has asubstantially circular bracket member 8 is provided at its peripheralportion with a flange 10 which extends radially outward therefrom. Thebracket 4 is fixed at a predetermined position as the flange 10 issecured by screws (not shown) to a base plate 12 of the magnetic diskdrive device.

A shaft member 14 is fixed to the center of the bracket 6. The shaftmember 14 maybe formed from, for example, a ferromagnetic material suchas a carbon steel. The shaft member 14 is press-fitted at its lower endto a hole formed in the bracket member 4. A stator 16 is secured to thecenter of the shaft member 14. The stator 16 has a stator core 18 whichis formed of a laminate of plates and an armature coil 20 which is woundon the stator core 18 in a predetermined fashion.

The shaft member 14 is provided with an axial through hole 22 throughwhich lead wires 24 from the armature coil 20 are led to the exterior ofthe bracket 4, i.e., to the exterior of a disk chamber 26.

Bearing members 28 and 30 are provided on both ends of the shaft member14. The hub 6 is rotatably supported through the intermediaries of theseshafts 28 and 30. The hub 6 has a cylindrical hub member 32 one end(lower end in the illustrated embodiment) of which is provided with aflange 34 which extends radially outward therefrom. An end wall 36 isprovided on the other end (upper end) of the hub member 32. The hub 6may be formed from aluminum and its upper end rotatably supports the endwall 36 through the bearing 28, while the lower end of the hub 6 isrotatably supported through a later-mentioned yoke member 38, an annularbush 40 and the above-mentioned bearing member 30.

The yoke member 38, which has a ring-like form, is mounted on the innerperipheral surface of the hub member 32. A rotor magnet 42 is mounted onthe upper end portion of the inner peripheral surface of the yoke member38.

The rotor magnet 42 is disposed so as to oppose the stator 16 which isfixed to the bearing member 14. The hub 6 is rotatingly driven by theinteraction between the rotor magnet 42 and the stator 16.

A stack of a plurality of magnetic disks 44, four disks in theillustrated embodiment, is mounted on the outer surface of the hub 6.These disks 44 are integrally fixed to the hub 6 by means of a clampmember 48 which is fastened to the hub 6 by means of screws 50 screwedinto threaded holes 52 in the hub 6.

A detailed description of the clamp member 48 and associated parts willbe given later.

A magnetic fluid seal 54 is provided on the outer side of one 28 of thebearing members. The magnetic fluid seal 54 has a magnetic circuit unitand a magnetic fluid 62. The magnetic circuit unit includes adoughnut-shaped magnet 56 and ferromagnetic pole pieces 58, 60 betweenwhich the magnet 56 is located. The doughnut-shaped magnet 56 ismagnetized such that N and S poles, for example, appear on both axialends thereof, so as to act on the ends of the pole pieces 60, 62,whereby the magnetic fluid 62 is held in the gaps between the ends ofthe pole pieces 60, 62 and the shaft member 14. The magnetic fluid seallayers formed between the pole pieces 60, 62 and the shaft member 14provide a partition between the bearing member 28 and the disk chamber26 accommodating the magnetic disks 44, thereby preventing lubricant inthe bearing member 28 and contaminant air stagnant in the hub 6 fromflowing and scattering into the disk chamber 26.

A labyrinth-type seal mechanism 64 is provided on the outer side of theother bearing member 30, i.e., in the space between the bearing member30 and the wall of the disk chamber 26. The labyrinth-type sealmechanism 64 is composed of annular recesses 66 formed in the bracketmember 8 and mating annular projections 68 provided on the lower endsurface of the flange 34 of the hub 6.

In the illustrated embodiment, an annular wall 70 projects axiallyoutward from the end wall 36 of the hub 6. The above-mentioned magneticcircuit unit 54 is mounted on the inner peripheral surface of thisannular wall 70. A cap 72 is disposed on the outer side of the magneticcircuit unit 54. The outer peripheral edge of the cap 72 fits on theinner peripheral surface of the annular wall 70. The clamp member 48 hasa disk-shaped clamper 74 which is provided at its outer peripheral edgewith a clamping portion 76 which acts on the magnetic disk 44. Theclamper 74 also is provided on the inner peripheral edge thereof with anannular shield member 78 which extends in the direction opposite to theclamping portion 76, i.e., radially outward. The clamp member 48 is madeof a ferromagnetic material.

The clamp member having the described construction is mounted on the hub6 in a manner shown in FIG. 1. In this state, the annular shield portion78 is located at the radially outer side of the annular wall 70 on thehub 6 so as to surround the entire periphery of the same. As aconsequence, the magnetic fluxes 80 from the magnetic circuit unit 54permeate through the annular wall 70 on the hub 6 and are converged onthe annular shield portion 78 of the clamp member 48.

Consequently, the magnetic fluxes are prevented from radially divergingbeyond the annular shield portion 78 into the region where the disks 44are disposed. It is therefore possible to prevent formation of amagnetic field by leaking magnetic fluxes B in the space where amagnetic head (not shown) operates, thus eliminating any undesirableeffect of the leaking magnetic fluxes B on the reading/writingcharacteristics of the magnetic head. The annular shield portion 78 alsofunctions as a guide for the clamp member 48 when the later is mountedon the hub 6, whereby the centering of the clamp member 48 isfacilitated.

FIG. 2 shows a second embodiment of the magnetic disk drive device ofthe present invention. For the purpose of an easier understanding, thesame reference numerals are used in FIG. 2 to denote the same parts ormembers as those in the first embodiment.

The hub 6 is made from aluminum or an aluminum alloy and is rotatablysupported by the shaft member 14 through bearing members 28 and 30. Thehub 6 has an end wall 36 on the inner peripheral portion of which isformed wall 70 which projects axially outward. The end wall 36 isprovided with an annular recess 90 formed in a portion thereof which ison the radially outer side of the wall 70. The annular recess 90 extendsaxially inward.

The clamp member 48 is made of, for example, a ferromagnetic materialsuch as an electromagnetic steel sheet, and is provided on the outerperipheral end thereof with a clamping portion 76a which acts on themagnetic disk 44. The clamp member 48 also is provided on the innerperipheral end thereof with an annular shield portion 35 formagnetically shielding magnetic fluxes as will be described later. Theclamp member 48 is mounted to the hub 6 by means of screws 50 which aredriven into threaded holes in the hub 6 through holes formed in theclamp member 48, whereby the magnetic disks 44 are fastened to the hub 6in a predetermined manner, as shown in FIG. 2. In this state, theannular shield portion 78 is located within an annular recess 90provided in the portion of the hub 6 around the wall 70, so as toproduce the following effect.

Magnetic fluxes 80 from the magnetic circuit unit of the magnetic fluidseal 54 permeate through the wall 70 so as to be converted in a magneticcircuit which is formed by the annular shield portion 78 which serves asa magnetic seal. As a consequence, the magnetic fluxes are materiallyprevented from reaching the disks 44, whereby the same advantages thatproduced by the first embodiment can be attained. The annular shieldportion 78 extends inward in the direction of axis of the hub 6, so thatthe height of the hub 6 can be appreciably reduced as compared with thefirst embodiment and the effective lengths of the threaded holes in thehub member 6 can be substantially increased for the screws 50.

The second embodiment also has a labyrinth-type seal mechanism which iscomposed of annular projections 94 formed on the inner peripheralsurface of the bracket member 8 and mating annular recesses 96 formed onthe outer (lower) surface of the bush 40 over the entire periphery ofthe latter. In addition, the lower surface 98 of the flange of the hub 6is disposed in the close proximity of the inner surface 100 of thebracket 4 so as to form a seal which effectively prevents oil mist orother contaminants in the bearing member 30 from coming into the diskchamber 26.

In the embodiments shown in FIGS. 1 and 2, the annular shield portion 78of the clamp member 48 materially shields the magnetic fluxes 80 fromthe magnetic circuit unit, so that it suffices only to use aferromagnetic material at least for the annular shield portion 80. Theclamp member 48 and the annular shield portion 80 can have any desiredconfigurations, provided that such configurations do not hamper thedescribed advantages of the present invention.

It is also possible to form an annular magnetic shield member separatefrom the clamp member 48 and to fit it is the annular recess 90 formedin the hub 6.

The second embodiment also incorporates the following featuresconcerning the pair of bearing members 28, 30.

In the second embodiment, the upper bearing 28 is a ball-type bearing.Since the magnetic fluid seal 54 is disposed in the vicinity of thisbearing 28, the viscosity of the lubricant charged in this bearing iscomparatively low. More specifically, the lubricant used in this bearing28 preferably has a viscosity level ranging between 10 and 30 cst at 40°C. For instance, a lubricant available under a commercial name MULTEMPSRL from Kyodo Yushi Kabushiki kaisha, as well as a lubricant sold asISOFLEX SUPER LDS 18 from KLUBEWR LUBRICATION MUNCHEN KG, can be usedsuitably. The lower viscosity of the lubricant used, the greater thetendency for scattering of the lubricant and formation of oil mist. Inthe second embodiment, however, the risk for the disk chamber 26 to beinvaded by such an oil mist can be eliminated by virtue of the presenceof the magnetic fluid seal 54 which effectively prevents the oil mistfrom coming into the disk chamber 26.

Thus, the magnetic fluid seal affords a greater allowance for theviscosity of the lubricant to be used in the upper bearing member 28,permitting the use of a lubricant which would minimize the friction and,hence, loss of torque.

The lower bearing 30 also is of the ball type. A lubricant having acomparatively high viscosity is charged in this bearing 30 because theseal mechanism 64 provided in the vicinity thereof is of labyrinth type.More specifically, the viscosity of the lubricant used in this bearinghas a viscosity level ranging from 80 to 110 cst at 40° C. A lubricantsold under commercial name of ANDOC C from Esso Standard Oil cansuitably be used as the lubricant for the bearing 30. The labyrinth-typeseal mechanism allows a slightly flow of air through the minute gapbetween the annular projections 94 and the annular recesses 96, so thatany mist of lubricant suspended by the air may pass through this sealmechanism together with the air. In order to minimize such leak oflubricant mist, it is necessary to use a lubricant having acomparatively large viscosity in the bearing 30. The use of such aviscous lubricant suppressed scattering of the lubricant and enhancesthe effect of the labyrinth seal.

Thus, in the second embodiment of the magnetic disk drive device of thepresent invention, a magnetic fluid seal 54 is used in combination withthe bearing 28 which is charged with a comparatively less-viscouslubricant, while a labyrinth-type seal mechanism 64 is used incombination with the bearing 30 which is charged with comparativelyviscous lubricant.

Although in the described second embodiment the magnetic fluid seal 54is disposed on the upper side of the hub 6 while the labyrinth-type sealmechanism 64 is disposed at the lower end of the hub 6, this is onlyillustrative and the arrangement may be reversed such that the upperbearing 28 is charged with a comparatively viscous lubricant to enablethe use of a labyrinth-type seal mechanism at the upper side of the hub6, while the lower bearing 30 is charged with a comparativelyless-viscous lubricant for cooperation with the a magnetic fluid sealused as the lower sealing means.

SPINDLE MOTOR

FIGS. 3 and 4 show an embodiment of the spindle motor of the presentinvention suitable for use in the magnetic disk drive device of thepresent invention.

The spindle motor shown in these Figures has a bracket member 103 and ahub 104. The bracket member 103 is provided with a circular hole formedsubstantially at the center thereof. A hollow cylindrical wall 106protrudes from the inner peripheral edge defining this circular hole,and a stator 108 is mounted on the outer peripheral surface of thiscylindrical wall 106. The stator 108 includes a stator core 112 fixed tothe cylindrical wall 106 and a coil 110 wound on the stator core 112.

The bracket member 103 mounts a bearing means 116 which will be detailedlater.

The hub 104 is an integral member having a cylindrical portion 130, anend wall 132 on the top of the cylindrical portion 130 and a shaftportion 134 on the center of the end wall 132. The shaft portion 134 isprovided with a threaded clamp hole 136 into which a screw (not shown)is driven to fasten a clamp member (not shown), whereby recording diskssuch as magnetic disks are secured to the hub 104.

The cylindrical portion 130 of the hub 104 covers the stator 108 whichis mounted on the cylindrical wall 106 of the bracket member 103, and arotor magnet 135 is fitted on the inner peripheral surface of thecylindrical portion 130. The arrangement is such that the hub 104 and,hence, the recording disks (not shown) mounted thereon are rotatinglydriven in a predetermined direction due to an interaction between therotor magnet 135 and the stator 108.

FIG. 4 is a sectional view of the bearing means 116. The illustratedbearing means 116 has a hollow cylindrical outer race (bearing sleeve)118, and an inner race 119 disposed inside the outer race 118. Pairs ofannular grooves 120, 121 are formed in the inner peripheral surface ofthe outer race 118 and the outer peripheral surface of the inner race119, such that a plurality of ball passages, upper and lower ballpassages in the illustrated case, are defined by the opposing annulargrooves. These passages are charged with a plurality of balls 122 whichcan roll and run along the annular grooves 120, 121. Thus, the outerrace 118 and the inner race 119 are rotatable relative to each otherthrough the action of the balls 122. The axial spacing between theannular grooves 120 in the outer race 118 and the axial spacing betweenthe annular grooves 121 in the inner race 119 are slightly differentfrom each other so that the balls charged between the outer and innerraces 118, 119 are suitably pre-loaded to exclude any play or rattleduring the operation.

An annular seal member 123 is fixed to the upper end of the outer race118. This seal member 123 has an inside diameter slightly smaller thanthe outside diameter of the inner race 119, so that the inner race 119does never contact with the seal member 123 during rotation relative tothe outer race 118. An annular seal member 124 is fixed to the lower endof the outer race 118. This seal member 124 also has an inside diameterslightly smaller than the outside diameter of the inner race 119 so thatit is never contacted by the inner race 119 when the latter rotatesrelative to the outer race 118. These seal members 123, 124 effectivelyprevent grease or other particles from scattering outside.

This bearing means 116 can be fabricated as a unit independently ofother components of the spindle motor and can easily be mounted in thespindle motor.

Referring again to FIG. 3, the outer race 118 of the bearing means 116is fixed to the hollow cylindrical wall 106 of the bracket member 103 bybonding, press-fitting or other suitable means. On the other hand, theinner race 119 of the bearing means 116 is fixed to the shaft portion134 of the hub 104 by bonding, press-fitting or other suitable means asis the case of the outer race 118.

The spindle motor having the described construction can be assembled bythe following method. The bearing means 116 as a unit as shown in FIG. 4is fabricated and assembled beforehand. The bearing means 116 is thenfixed by bonding, press-fitting or other suitable measure to the bracketmember 103 on which components such as the stator 108 have been mounted.Subsequently, the shaft portion 134 of the hub member 104 is bonded,press-fitted or likewise fixed to the bearing means 116. It is thuspossible to easily assemble the spindle motor. In addition, a highassembly precision is attained by virtue of the fact that the shaftmember 134 is integrally formed on the hub 104.

In the embodiment shown in FIGS. 3 and 4, the outer race 18 and theinner race 119 of the bearing means 116 are respectively fixed to thebracket member 102 and the hub 104. This, however, is not exclusive andthe arrangement may be such that the outer race 118 and the inner race119 of the bearing means 116 are respectively fixed to the hub 104 andthe bracket member 103, as in the case of a modification shown in FIG.5. In FIG. 5, the same reference numerals are used to denote the sameparts or components as those appearing in FIGS. 3 and 4, and detaileddescription of such parts or members is omitted.

Referring to FIG. 5, the bracket member 102 is provided with a shaftportion 134 projecting from substantially central portion thereof. Theshaft portion 134 is fixed to the inner race 119 of the bearing means116. A stator 108 is mounted on the outer peripheral surface of thecylindrical wall 106. The bearing means 116 may be of the same type asthat shown in FIG. 2.

On the other hand, the hub 104 is provided with a central hole 140 and acylindrical wall 142 around the central hole 140. Numeral 144 denotes aclamp hole. The cylindrical wall 142 is fixed to the outer race 118 ofthe bearing means 116.

According to the arrangement shown in FIG. 5, a high precision ofassembly is ensured by virtue of the fact that the shaft portion 134 isformed integrally with the bracket member 103.

The hole 140 may be covered by a seal member when the height of theshaft portion 134 is small.

In the arrangement shown in FIG. 5, a labyrinth-type seal mechanism isdisposed outside the bearing means 116. More specifically, a cap 148 ismounted on the inner peripheral surface of the cylindrical wall 142 ofthe hub 104 such that it cooperates with the shaft portion 140 informing a seal which effectively contributes to prevention of invasionof the disk chamber by the lubricant.

What is claimed is:
 1. A magnetic disk drive device, comprising:aspindle motor for rotatingly driving magnetic disks in a predetermineddirection, said spindle motor including a hub rotatably supportedthrough bearing means and having a portion for mounting said magneticdisks, a rotor magnet carried by said hub, a stator arranged to opposeand cooperate with said rotor magnet, and magnetic fluid seal means forpreventing scattering of lubricant from said bearing means; and clampmeans for securing said magnetic disks to said hub of said spindlemotor; wherein said hub has an end wall which is provided in the outersurface thereof with an annular recess at a portion corresponding to theposition of said magnetic fluid seal means, and said clamp means isprovided with an annular shield portion disposed in said annular recessand forming a shield against magnetic fluxes from said magnetic fluidseal means.
 2. A magnetic disk drive device, comprising:a spindle motorfor rotatingly driving magnetic disks in a predetermined direction, saidspindle motor including a hub rotatably supported through bearing meansand having a portion for mounting said magnetic disks, a rotor magnetcarried by said hub, a stator arranged to oppose and cooperate with saidmagnet, and magnetic fluid seal means for preventing scattering oflubricant from said bearing means and clamp means for securing saidmagnetic disks to said hub of said spindle motor; wherein said hub hasan end wall provided with an axially projecting wall formed at a portionthereof corresponding to the position of said magnetic fluid seal means,and said clamp means is provided with an annular shield portion disposedon the radially outer side of said projecting wall and forming a shieldagainst magnetic fluxes from said magnetic fluid seal means.